Not rocket science

: : That's a good question. But I don't have an answer. Rocket ship was first used in 1927 and rocket plane/airplane/aeroplane in 1928 when a rocket-powered airplane was only a "theoretical possibility." (“20th Century Words: The Story of New Words in English Over the Last 100 Years” by John Ayto. Oxford University Press, New York, 1999, Page 169-170.) Another reference says "rocket scientist" was used in the 1990s on United States campuses, meaning stupid. Ironic use of Standard English. (“Cassell’s Dictionary of Slang” by Jonathon Green, Wellington House, London, 1998. Page 1005.)

: [By the late 1960s, at some US high schools you could hear of a student who exhibited thick-headedness being called a rocket scientist because, according to the punchline, he was "taking up space." - Bac.]